Items Needed for Dorin Obi: Yellow Chip, Silver Obi, 3 Luminian Tissues, 7 Hpemde Organs, 3 Aern Organs,
Activating the Effects: The percents represents the % increase to the damage of a spell or the resistance rate down of a enfeebling spell.
Oppropriate day(10%): Granted by wearing Dorin's Obi on Earthsday
Single Weather Present(10%): Granted by wearing Dorin's Obi when earth weather is present.
Double Weather Present(20%): Granted by wearing Dorin's Obi when double earth weather is present.
So if it was Earthsday and there was Double weather present your spells would do 30% more damage or get resisted 30% less.
By the sounds of it, the effects given by the obi's are in all ways similar to any of the elemental staves so i'm assuming enhancing/healing/bard party buffs are unaffected.
Note that like the elemental staves these percentages are not set in stone but generally accepted.